1. Field of the Invention
The invention relates to hydraulic vibrators of the type that are adjustable as to optimum frequency range by adjustment of the volume of cylinder displacement and, more particularly, but not by way of limitation, it relates to an improved hydraulic vibrator with double rod end piston and reaction mass that includes structure to vary the internal cylinder volume in proportion to output frequency by varying the axial dimension of the cylinder chamber.
2. Description of the Prior Art
The prior art includes several different types of hydraulic vibrator wherein an attempt is made at controlled variation of cylinder volume in order to best utilize a mechanical vibrator at frequencies most in tune with its natural harmonics. U.S. Pat. No. 4,106,586 entitled "Hydraulic Vibrator" presents an early teaching wherein the hydraulic cylinder was varied in accordance with positioning of first and second end sleeves within the cylinder. In this case, the end sleeves were hydraulically movable between two positions by an ancillary hydraulic system. Following that, U.S. Pat. No. 4,388,981 was issued entitled "Variable Cylinder Hydraulic Vibrator and Control System" and this patent taught another hydraulic vibrator wherein the volume of the internal drive cylinder was varied by controlled positioning of opposite end sleeves; however, this teaching differed from the prior patent in that a continous or incremental adjustment was possible.
Another patent teaching is presented in U.S. Pat. No. 4,450,928 entitled "Dual Mode Seismic Source Vibrator" which automatically provides a shorter stroke for higher frequencies and longer stroke for lower frequencies by virtue of movable sleeves positioned within opposite ends of the drive cylinder. Here again, the sleeves are movable by application of fluid pressure in a manner similar to the previously discussed U.S. Pat. No. 4,388,981. Finally, U.S. Pat. No. 4,641,725 discloses a still further development wherein screw concepts are utilized to alter the effective lengths of the hydraulic cylinder chamber. The incremental variation of cylinder volume is achieved by synchronous rotation of threaded end sleeves that are disposed in opposite ends of the cylinder chamber. Several variations are disclosed but each requires synchronism of opposite end rotations.